1. Field of the Invention
The present invention relates to Planar Inverted-F Antenna (PIFA) and, in particular, to a method of designing a single and multi-band PIFA with a single feed.
2. Description of the Related Art
The cellular communication industry has experienced an enormous growth in recent years. Of late there has been an increasing emphasis on internal antennas for cellular handsets instead of a conventional external wire antenna. The conventional external wire antenna on a cellular handset exhibits an Omni directional radiation pattern in the azimuth plane. This results in a portion of transmitted power being lost by absorption into the user""s head and consequently leads to a higher value of Specific Absorption Rate (SAR). Internal antennas have several advantageous features such as being less prone for external damage, a reduction in overall size of the handset with optimization, easy portability, and potential for low SAR characteristics. The concept of internal antenna stems from the avoidance of protruding external radiating element by the integration of the antenna into the handset. The printed circuit board of the cellular handset serves as the ground plane of the internal antenna, and also acts to shield RF energy from user""s head. This shielding/blockage effect reduces the power radiated in the direction of the user""s head resulting in an improvement in the front to back (F/B) ratio of the radiation pattern of the internal antenna and lower value of SAR. Among the various choices for cellular internal antennas, PIFA appears to have great promise. The PIFA is characterized by many distinguishing properties such as being relatively lightweight, ease of adaptation and integration into the phone chassis, moderate range of bandwidth, Omni directional radiation patterns in orthogonal principal planes for vertical polarization, versatility for optimization, and multiple potential approaches for size reduction. A possible placement for PIFA inside a typical cellular handset to function as an internal antenna is shown in FIG. 10. The PIFA also finds useful applications in diversity schemes. Its sensitivity to both the vertical and horizontal polarization is of immense practical importance in mobile cellular communication applications because the antenna orientation is not fixed. All these features render the PIFA to be a good choice as an internal antenna for mobile cellular handsets. Despite all of the desirable features of a PIFA, the PIFA has the limitation of a rather large physical size for practical application. A conventional PIFA should have the semi-perimeter of its radiating element (sum of the length and the width) equal to xc2xc of a wavelength at the desired frequency. One-quarter of a wavelength at the center of AMPS frequency band (824-894 MHz) is 87.31 mm while the corresponding value at the center of GSM frequency band (880-960 MHz) is 81.52 mm. With the rapidly advancing size miniaturization of the cellular handset, the space requirement of a conventional PIFA is a severe limitation for practical application. Thus, there is a need for an efficient design technique to reduce the size of the PIFA, in order to realize a practical utility of the PIFA for cellular frequency bands.
Rapid expansion of the cellular communication industry in the recent past has created a need for multi-frequency band operation cellular handsets to meet the ever-increasing subscriber demand. In a typical multi-frequency band cellular handset with a single Duplexer, a multi-frequency band antenna with a single feed is the most viable option. Few attempts have been made in the past to design multi-frequency band PIFA with a single feed due to the complexity of design and difficulty in achieving acceptable bandwidths for the resonant bands desired. Multi-band PIFA designs have been realized in the past by using a separate feed path for each band. There is a great concern for a multi-band PIFA design with multiple feed paths having its performance compromised due to the mutual coupling and poor isolation of the various resonant bands. Therefore, the multi-band PIFA with multiple feed paths has not been a logical choice for practical applications in multi-frequency band cellular operations. Therefore, the design of single feed multi-band PIFA has been a topic of specific emphasis and special relevance to cellular communication.
A typical placement of a PIFA placed inside the housing of a typical cellular handset to function as an internal antenna is illustrated in FIG. 10. FIG. 10 is a schematic cut-away side view of a typical cellular handset 40 with an internal antenna 42. Cellular handset 40 includes a housing 41 in which antenna 42 and other accessories are enclosed. Among other things, the accessories of a cellular handset include a speaker 43, display 44, keypad 45, microphone 46, battery 47 and a printed circuit board 48 containing various electronic cards. Speaker 43 and microphone 46 define a user direction. When the cellular handset is in use with the keypad 45 pointing towards user""s head, the speaker 43 is placed in the vicinity of user""s ear and the microphone 46 is placed in the close proximity of the user""s mouth. In FIG. 10, the internal antenna 42 is placed directly over the printed circuit board 48 implying that the printed circuit board 48 also serves as a ground plane for the antenna 42. The internal antenna may also have a separate ground plane. In such a case, the ground plane of the internal antenna 42 is placed over the printed circuit board 48. The radiating element of the internal antenna 42 is oriented in a direction away from user""s head. The printed circuit board 48 which is located in the region between the internal antenna 42 and the user""s head, blocks a significant amount of the RF field radiated by the antenna 42 in the direction of the user""s head. Such a blockage effect offered by the printed circuit board 48 results in a dip or null in the radiation pattern of the antenna over an angular sector comprising the direction of the user""s head also. Consequently, the amount of RF power of the internal antenna 42 transmitted in the direction of the user""s head is considerably reduced resulting in low value of specific absorption rate (SAR).
A conventional prior art single band PIFA assembly is illustrated in FIGS. 11A and 11B. The PIFA 110 shown in FIG. 11A and FIG. 11B consists of radiating element 101, ground plane 102, connector feed pin 104a, and conductive post or pin 107. A power feed hole 103 is located corresponding to the radiating element 101. Connector feed pin 104a serves as a feed path for radio frequency (RF) power to the radiating element 101. The connector feed pin 104a is inserted through the feed hole 103 from the bottom surface of the ground plane 102. The connector feed pin 104a is electrically insulated from the ground plane 102 where the pin passes through the hole in the ground plane 102. The connector feed pin 104a is electrically connected to the radiating element 101 at 105a with solder. The body of the feed connector 104b is electrically connected to the ground plane at 105b with solder. The connector feed pin 104a is electrically insulated from the body of the feed connector 104b. A through hole 106 is located corresponding to the radiating element 101, and a conductive post or pin 107 is inserted through the hole 106. The conductive post 107 serves as a short circuit between the radiating element 101 and the ground plane 102. The conductive post 107 is electrically connected to the radiating element 101 at 108a with solder. The conductive post 107 is also electrically connected to the ground plane 102 at 108b with solder. The resonant frequency of the PIFA 110 is determined by the length (L) and width (W) of the radiating element 101 and is slightly affected by the locations of the feed pin 104a and the shorting pin 107. The impedance match of the PIFA 110 is achieved by the adjusting of the diameter of the connector feed pin 104a, by adjusting the diameter of the conductive shorting post 107, and by adjusting the separation distance between the connector feed pin 104a and the conductive shorting post 107. The fundamental limitation of the configuration of the PIFA 110 described in FIG. 11A and FIG. 11B is the requirement of relatively large dimensions of length (L) and width (W) of the radiating element 101 to achieve resonance in the desired cellular frequency bands (AMPS/GSM). This configuration is limited to only single operating frequency band applications.
The prior art techniques to reduce the physical size of the PIFA, while maintaining the resonance in the desired frequency bands include capacitive loading and dielectric loading. The dielectric loading increases the weight and cost of the PIFA while the capacitive loading in the prior art increases the mechanical complexity of the design, thus making it difficult and more expensive to manufacture. The details of these techniques are described below and are accompanied with illustrations. The elements of the PIFA configured with the capacitive loading and dielectric loading techniques which are similar to that of the conventional PIFA 110, will have the same reference numbers as in FIG. 11A and FIG. 11B. Therefore, additional redundant reference explanations have been omitted.
A prior art PIFA 120 with conventional capacitive loading is illustrated in FIGS. 12A and 12B. Plate 109 is placed parallel to the ground plane 102 and functions as a capacitive loading element for the radiating element 101. Plate 109 is separated from the ground plane 102 by a specific distance. The structural configuration of PIFA 120 with capacitive loading element as illustrated in FIG. 12A and FIG. 12B increases complexity and adds several steps to the manufacturing process. This results in an increased cost of this PIFA design.
A prior art PIFA 130 with conventional dielectric loading is illustrated in FIG. 13A and FIG. 13B. The entire area between the radiating element 101 and the ground plane 102 is filled with a block of dielectric material 110 of a specified dielectric constant. The introduction of the block of dielectric material into the antenna increases the weight and cost of the PIFA. The block of dielectric material 110 in the entire area of the PIFA also increases the dielectric loss and hence causing lower RF energy radiation efficiency.
A description of some prior art configurations of multi-band PIFA with multiple feeds and single feed is as follows. A prior art multi-band PIFA 140 with a separate feed for each band is illustrated in FIG. 14A, FIG. 14b and FIG. 14C. This configuration is a modification of the single band conventional PIFA 110 explained in FIG. 11A and FIG. 11B. As can be seen in FIGS. 14A, 14B and 14C, the multi-band PIFA 140 consists of two radiating elements 201a and 201b resonating at two separate frequency bands. The radiating elements 201a and 201b are positioned above a common ground plane 202. A narrow L-shaped slot 203 offers a physical division and electrically separates the two radiating elements 201a and 201b. A hole 204 is located corresponding to the radiating element 201a. A connector feed pin 205a, used for feeding radio frequency (RF) power to the radiating element 201a, is inserted through hole 204 from the bottom surface of the ground plane 202. The connector feed pin 205a is electrically insulated from the ground plane 202 where the pin passes through the hole in the ground plane 202. The connector feed pin 205a is electrically connected to the radiating element 201a at 206a with solder. The body of the feed connector 205b is connected to the ground plane at 206b with solder. The connector feed pin 205a is electrically insulated from the body of the feed connector 205b. A through hole 207 is located corresponding to the radiating element 201a. A conductive post or pin 208 which functions as a short circuit between the radiating element 201a and the ground plane 202 is inserted through the hole 207. The conductive post 208 is electrically connected to the radiating element 201a at 209a with solder. The conductive post 208 is connected to the ground plane 202 at 209b with solder. The radiating element 201a with relatively larger dimensions of length (L1) and width (W1) resonates at the lower frequency band of the multi-band operation.
The impedance match of the radiating element 201a is determined by the diameter of the connector feed pin 205a, the diameter of the conductive shorting post 208 and the distance of separation between the connector feed pin 205a and the conductive shorting post 208. The radiating element 201b with relatively smaller dimensions of length (L2) and width (W2) resonates at the higher frequency band of multi-band operation. A power feed hole 210 is located corresponding to the radiating element 201b. A connector feed pin 211a, used to feed radio frequency (RF) power to the radiating element 201b, is inserted through the feed hole 210 from the bottom surface of the ground plane 202. The connector feed pin 211a is electrically insulated from the ground plane 202 where the feed pin passes through the hole in the ground plane 202. The connector feed pin 211a is electrically connected to the radiating element 201b at 212a with solder. The body of the feed connector 211b is connected to the ground plane 202 at 212b with solder. The connector feed pin 211a is electrically insulated from the body of the feed connector 211b. A through hole 213 is located corresponding to the radiating element 201b. A conductive post or pin 214, which creates as a short circuit between the radiating element 201b, and the ground plane 202 is inserted through the hole 213. The conductive post 214 is electrically connected to the radiating element 201b at 215a with solder. The conductive post 214 is soldered to the ground plane 202 at 215b. The impedance match of the radiating element 201b is determined by the diameter of the connector feed pin 211a, the diameter of the conductive shorting post 214 and the distance of separation between the connector feed pin 211a and the conductive shorting post 214.
The configuration of multi-band PIFA 140 illustrated in FIG. 14A and FIG. 14B has several disadvantages. Such a configuration of the PIFA can be used only in a multi-band cellular handset with two Duplexers. However, the majority of currently manufactured cellular handsets have only one Duplexer. Adequate isolation between the two frequency bands requires a larger separation between the radiating elements 201a and 201b necessitating larger width of the L-shaped slot 203. The increased width of the L-shaped slot without increase of the overall dimensions of the radiating elements 201a and 201b reduces the bandwidth of the PIFA. Any change in the separation between the two resonant frequency bands involves the change of linear dimensions of the radiating elements 201a and 201b. 
Z. D. Liu, P. S. Hall and D. Wake, xe2x80x9cDual Frequency Planar Inverted-F Antennaxe2x80x9d, IEEE Trans. Antennas and Propagation, Vol. AP-45, No. 10, pp. 1451-1548, October 1997 (hereinafter referred to as Liu et al.) describes a multi-band PIFA with separate feeds with structural configuration similar to the one illustrated in FIG. 14A, FIG. 14B and FIG. 14c. P. Kabacik and A. A. Kuchaski, xe2x80x9cOptimising the Radiation Pattern of Dual, Frequency Inverted-F Planar Antennasxe2x80x9d, JINA Conference, pp. 655-658, 1998 (hereinafter referred to as Kabacik et al.) also describes a multi-band PIFA with separate feeds with similar configuration to the one illustrated in FIG. 14A, FIG. 14B and FIG. 14C. Instead of an L-shaped slot 203 separating the two radiating elements as in FIGS. 14A and 14B, a U-shaped slot has been proposed by Kabacik et al.
A prior art multi-band PIFA 150 with a single feed is illustrated in FIG. 15A and FIG. 15B. The multi-band PIFA 150 consists of a radiating element 301 and a ground plane 302. An L-shaped slot 303 on the radiating element 301 creates a quasiphysical partitioning of the radiating element 301. The segment on the radiating element 301 with dimensions of length (L1) and width (W1) resonates at the lower frequency band of the multi-band operation. The segment on the radiating element 301 with dimensions of length (L2) and width (W2) resonates at the upper frequency band of the multi-band operation. A power feed hole 304 is located corresponding to the radiating element 301. A connector feed pin 305a, used for feeding radio frequency (RF) power to the radiating element 301, is inserted through the feed hole 304 from the bottom surface of the ground plane 302. The connector feed pin 305a is electrically insulated from the ground plane 302 where the feed pin passes through the hole in the ground plane 302. The connector feed pin 305a is electrically connected to the radiating element 301 with solder at 306a. The body of the feed connector 305b is connected to the ground plane 302 at 306b with solder. The connector feed pin 305a is electrically insulated from the body of feed connector 305b. A through hole 307 is located corresponding to the radiating element 301. A conductive post or pin 308 which functions as a short circuit between the radiating element 301 and the ground plane 302 is inserted through the hole 307. The conductive post 308 is connected to the radiating element 301 at 309a with solder. The conductive post 308 is also connected to the ground plane 302 at 309b with solder. The multi-frequency band impedance match of the radiating element 301 is determined by the diameter of the connector feed pin 305a, the diameter of the conductive shorting post 308 and the separation distance between the connector feed pin 305a and the conductive shorting post 308. The main disadvantage of the configuration of the multi-band PIFA 150 illustrated in FIG. 15A and FIG. 15B is the lack of simple means of adjusting the separation of the lower and upper resonant frequency bands. The change in the separation of the resonant frequency bands requires the repositioning of the slot 303. Liu et al. describes a configuration of a single feed multi-band PIFA, which is similar to the one described in FIG. 15A and FIG. 15B. In the single feed multi-band PIFA configuration of Liu et al., the concept of dielectric loading illustrated in FIG. 13A and FIG. 13B has also been invoked.
In the first embodiment of the invention, the single feed multi-band PIFA is characterized by a radiating element located above the ground plane, a shorting pin or post along the centerline of the radiating element adjacent to the power feeding connector pin, a vertical loading plate on the radiating edge adjacent to the power feeding connector pin, a horizontal loading plate on the other radiating edge adjacent to the shorting post, and a block of dielectric material of a specific dielectric constant filling the area between the horizontal loading plate and the ground plane. In a second embodiment of the invention, PIFA is essentially the same as in the first embodiment except that in the second embodiment, a slot loading technique to adjust the resonant frequency of desired bands is described. A third embodiment of the invention is in the design of a single band PIFA having reduced dimensions of the radiating element including the concepts of slot loading, modified capacitive loading and partial dielectric loading combined therein.
One of the principal objects of the invention is to circumvent the use of separate feeds for the realization of multi-band operation of a PIFA.
A further object of the invention is to provide an efficient design method to achieve the multi-band operation of a PIFA using only a single feed path.
Still another object of the invention is to provide a single feed multi-band PIFA which is devoid of currently imposed physical partition of the original structure of a single band PIFA.
Still another object of the invention is to provide a design of a single feed multi-band PIFA which has the merit of relative ease of adjusting the separation between the resonant bands without necessitating a dimensional change of the radiating element.
Still another object of the invention is to provide a single feed multi-band PIFA configuration having the desirable features of configuration, simplicity, compact size, cost-effectiveness to manufacture and improved manufacturability.
Still another object of the invention is to provide a compact single band PIFA.
Still another object of the invention is to provide a design of the type described above which involves a combination of a modified prior art capacitive loading technique, a technique of partial dielectric loading and a technique of slot loading.
These and other objects will be apparent to those skilled in the art.